ObjCXX: fix a crash during typo correction.
For ObjCXX, we can create a CastExpr with Kind being CK_UserDefinedConversion
and SubExpr being BlockExpr. Specifically one can return BlockExpr from
BuildCXXMemberCallExpr and the result can be used to build a CastExpr.
Fix the assumption in CastExpr::getSubExprAsWritten that SubExpr can only